In a thermal bubble inkjet printing system, an inkjet printhead prints an image by ejecting ink droplets through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium move relative to each other. Thermal inkjet printheads eject droplets from a nozzle by passing electrical current through a heating element to generate heat and vaporize a small portion of the fluid within a firing chamber. The current is supplied as a pulse which lasts on the order of 2 micro-seconds. When a current pulse is supplied, the heat generated by the heating element creates a rapidly expanding vapor bubble that forces a small droplet out of the firing chamber nozzle. When the heating element cools, the vapor bubble quickly collapses, drawing more fluid from a reservoir into the firing chamber in preparation for ejecting another drop from the nozzle.
During printing, heat from the heating elements influences the temperature of the thermal inkjet die. The temperature of the thermal inkjet die has a significant influence on characteristics of the ink droplets being fired from the nozzles, and can therefore have an adverse impact on the overall print quality of the printing system. For example, a higher temperature in the die results in a higher drop weight and a higher drop velocity, while a lower die temperature results in a lower drop weight and velocity. Thus, variations in temperature across the die can result in droplets of different weight being ejected onto the print medium. Differences in the drop weight (and drop velocity, to a lesser degree) can have a considerable impact on the print quality. Drops with lower drop weight being ejected from a cooler area of the die can result in areas on the print medium having less ink than intended. The areas printed with less ink will appear to be lighter than other areas printed with drops of higher drop weight that are ejected from warmer areas of the die. Variations in drop weight can also adversely affect the color accuracy of the printing system. In general, the problems caused by variations in drop weight and velocity are referred to as light area banding (LAB), die boundary banding (DBB), and hue shift.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.